1. Field of the Invention
The present invention relates to a semiconductor device or an integrated circuit which operates at a high speed or at a high frequency such as at a microwave band, and to a method for producing the same.
2. Description of the Related Art
There are recent demands for transistors or integrated circuits which operate at a high speed or at a microwave band, and such transistors or integrated circuits are being intensively researched and developed.
FIG. 4 schematically shows a cross sectional view of a conventional field effect transistor (hereinafter referred to as FET) 100 operating at a microwave band. The FET 100 is formed on a semiinsulating gallium arsenide (hereinafter referred to as GaAs) substrate 101. An n-type GaAs layer 102 is formed on the GaAs substrate 101. The n-type GaAs layer 102 is doped with silicon as n-type impurities and functions as an active layer. A source electrode 105 and a drain electrode 106 are formed on the n-type GaAs layer 102 for ohmic contact. The n-type GaAs layer 102 has a recess 103 at a surface of the n-type GaAs layer 102. The recess 103 is positioned between the source electrode 105 and the drain electrode 106. A gate electrode 104 is formed on the recess 103, and the gate electrode 104 forms Schottky connection with the n-type GaAs layer 102. The entire surface of the FET 100 is covered with a passivation film 107 made of a material such as silicon oxide, phospho-silicate glass, silicon nitride, etc.
In the FET 100, a depletion region 108 is formed in a region of the n-type GaAs layer 102 under the gate electrode 104 and prevents a current from flowing between the source electrode 105 and the drain electrode 106. The thickness of the depletion region 108 is changed by a gate voltage applied to the gate electrode 104. Thus, the gate voltage can control the current between the source electrode 105 and the drain electrode 106. The thickness of the depletion region 108 should be changed as quickly as possible in accordance with the potential of the gate electrode so as to operate the FET 100 at a high frequency. It is necessary to reduce gate capacitance, particularly a capacitance between the gate electrode 104 and the source electrode 105 (hereinafter referred to as capacitance C.sub.gs) for improvement of device characteristics at a high frequency.
In the conventional FET 100 as is explained above, the gate electrode 104 is covered with the passivation film 107 to protect the FET 100 from humidity, dust, and chemicals causing atmospheric corrosion. However, the passivation film 107 has a larger dielectric constant, which increases the capacitance C.sub.gs Therefore, the FET 100 has a large capacitance C.sub.gs, which degrades device characteristics at the microwave band of the FET 100. Moreover, when the FET 100 is molded in a plastic package, the gate electrode 104 is in contact with the plastic package via the passivation film 107. Thus, the device characteristics are more degraded because of the high dielectric constant of the plastic package.
In view of the aforementioned shortcomings associated with conventional transistors, there is a need in the art for a transistor which has a small gate capacitance and good device characteristics at a high frequency.